Rosalba Saija

Nanopolaritons: vacuum Rabi splitting with a single quantum dot in the center of a dimer nanoantenna.

We demonstrate with accurate scattering calculations that a system constituted by a single quantum emitter (a semiconductor quantum dot) placed in the gap between two metallic nanoparticles can display the vacuum Rabi splitting. The largest dimension of the investigated system is only 36 nm. This nonperturbative regime is highly desirable for many possible applications in quantum information processing or schemes for controlling individual photons. Along this road, it will be possible to implement scalable photonic quantum computation without renouncing to the nanometric size of the classical logic gates of the present most compact electronic technology.

Brownian Motion of Graphene

Brownian motion is a manifestation of the fluctuation-dissipation theorem of statistical mechanics. It regulates systems in physics, biology, chemistry, and finance. We use graphene as prototype material to unravel the consequences of the fluctuation-dissipation theorem in two dimensions, by studying the Brownian motion of optically trapped graphene flakes. These orient orthogonal to the light polarization, due to the optical constants anisotropy. We explain the flake dynamics in the optical trap and measure force and torque constants from the correlation functions of the tracking signals, as well as comparing experiments with a full electromagnetic theory of optical trapping. The understanding of optical trapping of two-dimensional nanostructures gained through our Brownian motion analysis paves the way to light-controlled manipulation and all-optical sorting of biological membranes and anisotropic macromolecules.

Optical properties of spheres containing a spherical eccentric inclusion

The formalism presented a few years ago by Fikioris and Uzunoglu [ J. Opt. Soc. Am.69, 1359 ( 1979)] to describe the electromagnetic scattering by homogeneous spheres containing an eccentric spherical inclusion is reformulated. The resulting approach is an extension of our previous formalism [ Aerosol Sci. Technol.3, 27 ( 1984)] designed to deal with the dependent scattering by aggregated spheres and is put in a form readily extensible to the case of spheres containing more than one inclusion. A comparison of our results with those of Fikioris and Uzunoglu is made, and the differences are explained in terms of the approximations that they used. Specific results for dielectric spheres containing either a metallic inclusion or a dielectric inclusion with parameters quite incompatible with the approximation scheme of Fikioris and Uzunoglu are also presented. These scatterers have a response that depends on the direction of incidence and, in general, also on the polarization, thus making them distinguishable from spheres with a centered inclusion as well as from homogeneous spheres.

Plasmon-enhanced optical trapping of gold nanoaggregates with selected optical properties.

We show how light forces can be used to trap gold nanoaggregates of selected structure and optical properties obtained by laser ablation in liquid. We measure the optical trapping forces on nanoaggregates with an average size range 20-750 nm, revealing how the plasmon-enhanced fields play a crucial role in the trapping of metal clusters featuring different extinction properties. Force constants of the order of 10 pN/nmW are detected, the highest measured on a metal nanostructure. Finally, by extending the transition matrix formalism of light scattering theory to the optical trapping of metal nanoaggregates, we show how the plasmon resonances and the fractal structure arising from aggregation are responsible for the increased forces and wider trapping size range with respect to individual metal nanoparticles.

Optical trapping calculations for metal nanoparticles. Comparison with experimental data for Au and Ag spheres.

We calculate the optical forces on Au and Ag nanospheres through a procedure based on the Maxwell stress tensor. We compare the theoretical and experimental force constants obtained for gold and silver nanospheres finding good agreement for all particles with r < 80 nm. The trapping of the larger particles recently demonstrated in experiments is not foreseen by our purely electromagnetic theory based on fixed dielectric properties. Since the laser power produces a heating that may be large for the largest spheres, we propose a model in which the latter particles are surrounded by a steam bubble. This model foresees the trapping of these particles and the results turn out to be in reasonable agreement with the experimental data.

Optical trapping of nonspherical particles in the T-matrix formalism

The theory of the trapping of nonspherical particles in the focal region of a high-numerical-aperture optical system is formulated in the framework of the transition matrix approach. Both the case of an unaberrated lens and the case of an aberrated one are considered. The theory is applied to single latex spheres of various sizes and, when the results are compared with the available experimental data, a fair agreement is attained. The theory is also applied to binary clusters of spheres of latex with a diameter of 220nm in various orientations. Although, in this case we have no experimental data to which our results can be compared, we get useful indications for the trapping of nonspherical particles. In particular, we find substantial agreement with recent results on the trapping of prolate spheroids in aberrated gaussian fields [S. H. Simpson and S. Hanna, J. Opt.Soc. Am. A 24, 430 (2007)].

Optical properties of spheres containing several spherical inclusions

The formalism that was previously devised [J. Opt. Soc. Am. A 9, 1327 (1992)] to deal with the optical properties of homogeneous spheres containing an eccentric spherical inclusion is extended to the case of several inclusions. The extinction efficiency of dielectric spheres containing two identical metallic inclusions is calculated for a few significant geometries. Extinction by a low-density dispersion of the anisotropic scatterers mentioned above is also evaluated. Our results show that the subdivision of the included material has quite visible effects that strongly depend on both the polarization of the incident light and the geometric arrangement of the inclusions.

Rotational dynamics of optically trapped nanofibers.

We report on the experimental evidence of tilted polymer nanofiber rotation, using a highly focused linear polarized Gaussian beam. Torque is controlled by varying trapping power or fiber tilt angle. This suggests an alternative strategy to previously reported approaches for the rotation of nano-objects, to test fundamental theoretical aspects. We compare experimental rotation frequencies to calculations based on T-Matrix formalism, which accurately reproduces measured data, thus providing a comprehensive description of trapping and rotation dynamics of the linear nanostructures.

Size-scaling in optical trapping of silicon nanowires.

We investigate size-scaling in optical trapping of ultrathin silicon nanowires showing how length regulates their Brownian dynamics, optical forces, and torques. Force and torque constants are measured on nanowires of different lengths through correlation function analysis of their tracking signals. Results are compared with a full electromagnetic theory of optical trapping developed in the transition matrix framework, finding good agreement.

Optical properties of a sphere in the vicinity of a plane surface

The full scattering pattern from a sphere in the vicinity of a plane surface is calculated through an approach based on the expansion of the electromagnetic field in terms of vector multipole fields and on the imposition of the boundary conditions. Our approach does not invoke any approximation but can easily incorporate the simplifying assumptions of Bobbert Vlieger [Physica (Utrecht)137A, 202 (1986)] and of Johnson [J. Opt. Soc. Am. A13, 326 (1996)], whose results are compared with ours. Real progress is achieved, since, unlike the previous theories but in agreement with the available experimental data, a nonvanishing field is allowed to propagate along the surface, even when the latter is nonperfectly reflecting.